Most of the post above is not very accurate. Bitcoin is defined as one of many "decentralized crypto-currencies".
It's essentially the time that is estimated to take to solve a very complex computer algoryrthm. The more complex the algoryrthm, the more valuable the crypto currency or coin. The value of any crypto currency is actually predetermined because the time that it will take to solve it's algorithm is known long before anyone tries to solve such problems.
As people "mine" bitcoin or solve those algorythims, they become more valueave because there is a predetermined number of solutions. So, as time goes on, fewer and fewer coins will be available to "mine".
It's the value vs. OTHER centralized currencies like the US dollar that changes quite a bit. The relative value of all other crypto-currencies are almost always compared against bitcoin and that in turn is compared to the USD and all other currency exchange rates.
That's the basics but you can simply look at the Wikipedia page to get the gist of its origins and the professor that came up with the idea. It's quite interesting to me but I'm a computer nerd so you may not really find it that fascinating if you are not working in that industry.
